Generally, a constant temperature box used for a picnic or fishing comprises inner and outer casings of stainless steel, which each comprise a square bottom and four side walls rising from four side edges of the bottom and are laid one on the other. Interposed between the casings is an adiabatic material mainly of foam styrol.
Such constant temperature box using the inner and outer casings of expensive stainless steel is not only larger in weight but also expensive to produce, and also requires three constituents of inner and outer casings and adiabatic material to thus increase the number of parts and take much time for machining and assembly, thereby increasing the manufacturing cost. Furthermore, the box not-unfolded is bulky for storing or transportation to result in a great expense therefor.